This invention relates to a novel method for vaporizing getter material inside a vacuum electron tube and particularly, but not exclusively, to a novel method for flashing the getter material from a getter container in a color television tube.
In one popular design of a color television picture tube, which is a type of cathode-ray tube, a getter container having getter material therein is held against or close to the inner surface of the envelope, usually that part of the envelope called the funnel. After the envelope is evacuated of gases and sealed, an induction coil is positioned against or close to the outer surface of the envelope opposite the getter container and is then energized with a high-frequency current. The magnetic field generated by the energized coil induces currents in the getter container causing the temperature of the getter container and the getter material therein to rise rapidly until getter material, which is usually barium metal, vaporizes or "flashes" and deposits as a getter film on internal surfaces of the tube. A purpose of the getter film is to absorb both residual gas left in the envelope after evacuation and adsorbed gas that is later evolved from internal surfaces during the operating life of the tube. The life of the tube is determined principally by the ability of the getter film to continue to absorb gas and to maintain a low gas pressure in the envelope.
In order to vaporize the maximum amount of getter material from the container and to realize a desired distribution of deposited getter material in the tube, it is necessary to position the induction coil properly with respect to the getter container to produce optimum magnetic coupling between them. This is not easily done. Although the envelope is usually constituted of a transparent glass, the getter container cannot be seen (optically) from outside the tube because the inner surface of the envelope opposite the getter container is coated with an opaque conductive coating.
Heretofore, it was the common procedure to make a dummy tube without any opaque wall coating present, and then to determine where the induction coil should be located on tubes of that design in order to flash the getter material from the getter container. Since, during factory production, there is some variation from the nominal position of the getter container, this prior procedure results in a corresponding variation in the amount and uniformity of the deposited getter material. To compensate for misalignment between the induction coil and the getter container, a large, flat "pancake" coil is used. However, while the use of a large, flat coil insures the flashing of the getter material, it never creates the uniform heating in the getter container for realizing the best control of the getter-flashing method.